Conventionally, there are known ultrasonic sensors or ultrasonic switches used for detection of objects or other purposes. They are disclosed in, e.g., Japanese Utility Model Laid-open Publication Nos. S62-163781 and S62-140378. The ultrasonic elements disclosed in these prior art documents are covered with elastic members for an anti-vibration purpose and are so-called closed ultrasonic elements that generate ultrasonic waves by the vibration of an element body itself. In addition to the closed ultrasonic elements, there are known so-called open ultrasonic elements in which an ultrasonic oscillator is arranged within an element body with an opening on one surface so that ultrasonic waves generated by the oscillator can be transmitted through the opening. A conventional ultrasonic sensor using the open ultrasonic elements will now be described with reference to FIGS. 7A and 7B. In the following description, the direction running toward the upper and lower sides in FIG. 7A is defined as an up-down direction.
Referring to FIGS. 7A and 7B, the conventional ultrasonic sensor makes use of an ultrasonic element 100 that includes a cylindrical element body 100a having a bottom with an opening (not shown) for transceiving ultrasonic waves and an ultrasonic oscillator (not shown) arranged within the element body 100a to transceive ultrasonic waves. The ultrasonic element 100 is stored in a cylindrical receiving portion 200a having a bottom, which is provided in a rectangular box-shaped body 200 with an open top. The body 200 is coupled with a rectangular box-shaped cover 201 having an open bottom with the openings thereof facing each other. The body 200 and the cover 201 make up a housing 202.
A flat printed wiring board 203 is fixed to the body 200 by set screws 205. The printed wiring board 203 includes at least one of a drive circuit for driving the ultrasonic element 100 in response to a drive signal from outside and a converter circuit for converting ultrasonic waves received by the ultrasonic element 100 to a wave-receiving signal.
The ultrasonic element 100 is provided with a pair of terminal portions 100b on the upper surface of the element body 100a and is stored in the receiving portion 200a with the terminal portions 100b facing toward the opening of the body 200. The terminal portions 100b are soldered to the printed wiring board 203. In the bottom of the receiving portion 200a, there is formed a communication hole 200b communicating with the opening of the ultrasonic element 100 and the outside. The communication hole 200b serves as a horn that expands the wave-transceiving range of ultrasonic waves and decides the directivity of ultrasonic waves.
Since the element body 100a does not vibrate in the open ultrasonic element 100 described above, there is no need to use an elastic member for an anti-vibration purpose. However, when storing the ultrasonic element 100 in the receiving portion 200a, it is difficult to correctly position the ultrasonic element 100 due to the gap between the ultrasonic element 100 and the receiving portion 200a. This may make the center of the communication hole 200b and the center of the opening of the ultrasonic element 100 misaligned, which affects the directivity of ultrasonic waves. Thus, variations occur in the directivity of ultrasonic waves, possibly worsening the detection performance. For the reasons noted above, a cylindrical elastic member 204 is attached to the ultrasonic element 100 in the conventional ultrasonic sensor as shown in FIG. 7A, thereby filling up the gap between the receiving portion 200a and the ultrasonic element 100. As a result, the ultrasonic element 100 is brought into contact with the elastic member 204 under pressure and, therefore, is correctly positioned within the storage unit 200a. This makes it possible to solve the problems mentioned above.
In the conventional ultrasonic sensor, however, the step of attaching the elastic member 204 to the ultrasonic element 100 prolongs the manufacturing time of the ultrasonic sensor because the elastic member 204 needs to be attached to the ultrasonic element 100. Moreover, the manufacturing efficiency is reduced because the step of attaching the elastic member 204 to the ultrasonic element 100 is time-consuming. In addition, the manufacturing cost is increased because the elastic member 204 needs to be manufactured independently of the other members such as the ultrasonic element 100.